Minerals

What makes up a rock

Mining and Exploration

Geology Handbook

Minerals

Elements, as in the periodic table of, are the simplest substances made up of molecules. They cannot be broken down or easily converted into another element. Elements come from stars, all elements heavier than iron (Fe), come from supernovae. Everything is made of elements, We are made from stardust.

When elements combine to form more complex crystalline materials using natural processes, you get minerals. Minerals can be simply an element (gold – Au), or complex (Feldspar – CaAl2Si2O8).

Minerals are defined if and only if they are naturally occurring, crystalline in nature, have a defined chemical structure.

Rocks are combinations of one or more minerals and may contain other compounds with a complicated chemical structure.

The most common minerals

In order of abundance in the earths crust;

Plagioclase Feldspar

Alkali Feldspar

Quartz

Pyroxene

Amphiboles

Olivine

Micas

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Mineral varieties can vary by small amounts of impurities.

Identifying Minerals

Minerals are identified based on their physical properties, however some minerals and mineral varieties require more in depth identification methods.

Basic Mineral identification

Colour and Streak;

The base colour of the mineral as well as the colour left when it is scratched across a streak plate (white or black porcelain tile). Colour and streak are easy to identify however not overly indicative due to varying colours within a single mineral due to impurities to oxidation (rust) and even exposure to sunlight or biological material can change the colour.

Specific Gravity;

How much does it weigh, compared to a similar rock of similar size. A metallic mineral (pyrite) will be heavier than a silicate mineral (Quartz). Galena, a common lead ore has a very high specific gravity because of its lead composition. Pure gold will also have a very high specific gravity.

Hardness

The hardness of a mineral is measured on a scale from one to ten called the Mohs’ Hardness scale based on the ability of one mineral to scratch another. There are ten minerals in Mohs scale, from softest to hardest; talc, gypsum, calcite, fluorite, apatite, feldspar, quartz, topaz, corundum, and for last and hardest, diamond. A diamond will scratch everything, talc can be scratched by everything. Thens its just a matter of seeing what scratches what to determine the minerals hardness (which may vary slightly).

Lustre

How light reflects from the surface of the mineral both in its quality and intensity. Metallic and non-metallic is the first break point with lustre. Non metallic lustres include adamantine (diamonds), vitreous or glassy, pearly, resinous and silky.

Cleavage

Crystal structure or Habit

Other properties like magnetic, reacts with dilute acid, even smell help to identify some minerals and mineral groups. Sulfides like Pyrite smell like rotten eggs when struck/powdered, Sylvite is a very bitter tasting Salt, Carbonates react to dilute acid and Magnetite is well, magnetic

Classification: Large Crystals or small crystals?

The individual crystal size of all rocks cooling from magma or lava is a direct result of how much time they have to grow. The longer a mineral has to grow, the larger the crystals will be. This means the speed at which molten rock solidifies directly determines the size of the crystals.

So for a given magma composition, there will be an extrusive and an intrusive version, Chemically identical to each other. Basalt is the extrusive (from lava flows) version of Gabbro. Rhyolite is the extrusive version of Granite.

And just to get confusing, some magmas may cool quickly at one point, but slowly at another (two stage cooling) which will result in some large crystals in a mostly small crystal rock. These rocks and their texture are classified as porphyritic.

A magma that has millions of years to cool deep in the earth will tend to form/grow large easily distinguishable crystals. A magma (or lava at the earth’s surface) which explodes into air or water and cools very quickly will only have very tiny crystals – so small you may need magnification to see them.

Fractional crystallization.

There are also rocks where some crystals form and sink to the bottom of the magma, then another, and another… forming layers. This is especially important to the formation of chromite deposits.